I think you can, provided the person that you're instructing has their HP and is current... Since it's only am "endorsement" and not a "rating." In that case, the other guy would be acting as PIC. You're not teaching stuff about the airplane, you're teaching instruments. You don't need an HP to teach instruments.

That's how I've heard similar scenarios solved, but I could be (and usually am!) wrong.

I THINK you'd be able to log everything but PIC.

You'll certainly want to hear some more input first.